1. Field of the Invention
This invention relates to optical components, in which circumferential walls of optical transmission elements such as lenses partially join metal holders via joining materials, and metal holders for holding optical transmission elements.
This application claims priority on Japanese Patent Application No. 2003-328452, the content of which is incorporated herein by reference.
2. Description of the Related Art
Conventionally, optical devices (or optical components) are designed such that optical transmission elements such as lenses join metal holders via low melting point glasses. FIG. 5 shows an example of the aforementioned optical device, which is disclosed in Japanese Patent Application Publication No. H02-281201. An optical device 40 shown in FIG. 5 is produced using an optical transmission element 41 made of an optical lens having a thermal expansion coefficient of 120×10−7/° C., which is arranged inside of a metal holder 42 made of a stainless steel having a thermal expansion coefficient of 170×10−7/° C. and which then joins the metal holder 42 via a low melting point glass 43 composed of PbO—B2O3 having a softening point of 350° C. and a thermal expansion coefficient of 110×10−7/° C.
In the optical device 40 comprising the optical transmission element 41 and the metal holder 42, it is possible to replace the low melting point glass 43 with the adhesive or the alloy solder made of lead and tin. The adhesive generally has a high hygroscopic property, so that optical device 40 using the adhesive may become fragile in certain environmental conditions. In addition, the relatively low glass dislocation temperature reduces the use-allowable temperature range of the optical device 40 using the adhesive, and so-called “outgassing phenomenon” may occur. This causes a problem in that optical devices using the adhesive do not meet long-term reliability.
The alloy solder composed of lead and tin has a relatively low melting point; therefore, when a certain load such as gravity is normally applied to the soldered portion of an optical device, a creeping phenomenon in which the solder becomes deformed over time may easily occur. That is, when the optical transmission element 41 and the metal holder 42 are fixed together using the alloy solder, the position of the optical transmission element 41 may vary over time; therefore, it is very difficult to guarantee the stability of the optical system over a long time. In addition, the alloy solder has a thermal expansion coefficient of 250×10−7/° C., which greatly differs from the thermal expansion coefficient of 120×10−7/° C. of the optical transmission element 41.
Due to the aforementioned difference of the thermal expansion coefficients, when the optical transmission element 41 is fixed to the metal holder 42 via the alloy solder, a stress is applied to the optical transmission element 41 during the cooling of the alloy solder, which causes cracks and double refraction in the optical transmission element 41. Due to temperature variations or variations of surrounding temperature caused by heating of an electronic circuit and the like, tensile and compressive stresses may be repeatedly applied to the soldered portion. Furthermore, thermal fatigue may cause cracks in the solder so that the optical transmission element 41 may be varied in position and in optical axis. Because of the reasons described above, the optical device 40 is designed to use the low melting point glass 43.
The recent technology introduces vacuum evaporation in forming thin films composed of magnesium fluoride (MgF2), whereby antireflection films are formed on the light incoming surface and light outgoing surface of the optical transmission element (e.g., an optical lens) so as to avoid unwanted reflection of light, thus improving the transmittance. FIG. 6 shows an optical device 50 using an optical transmission element 51, which is improved in transmittance by forming an antireflection film 54 therewith. The optical device 50 of FIG. 6 is produced such that the optical transmission element 51 having the antireflection film 54 is arranged inside of the metal holder 52, and then it joins the metal holder 52 via a low melting point glass 53.
The antireflection film 54 has a heat resistance of 400° C. or less. However, it is difficult for the PbO—B2O3 material, which is normally used for the low melting point glass 53, to have a reduced burning temperature of 450° C. or less; that is, it is difficult for the burning temperature of the low melting point glass 53 to be decreased to be equal to the heat resistance temperature of the antireflection film 54 or less. In consideration of environmental protection, it is necessary for harmful materials such as PbO to be eliminated from the low melting point glass 53. However, the low melting point glass 53 has a surface (or surfaces) exposed to the atmosphere other than surfaces thereof joining the optical transmission element 51 and the metal holder 52. This may accelerate the deterioration of the low melting point glass 53 in high humidity environments.
Another type of a low melting point glass that can be subjected to burning at a relatively low temperature, which is lower than that of the low melting point glass 43 mainly composed of a lead glass, is disclosed in Japanese Patent Application Publication No. H08-259262. In addition, another type of an optical device in which an optical transmission element joins a metal holder by using a low melting point glass whose lead content is 0.1 weight % or less is disclosed in Japanese Patent Application Publication No. 2003-40648. Due to the constitution of the optical device in which the optical transmission element joins the metal holder by using the low melting point glass whose lead content is 0.1 weight % or less, the burning temperature can be decreased to 400° C. or less; thus, it is possible to improve the reliability in securing desired optical transmission characteristics. Since the low melting point glass has a relatively low lead content that is 0.1 weight % or less, it is possible to produce the optical device that is environmentally friendly to the earth.
However, due to the intervention of the low melting point glass between the optical transmission element (e.g., an optical lens) and the metal holder in the optical device (or optical component) disclosed in the aforementioned documents, there is a problem in that the incoming light incident on the optical transmission element is varied in optical characteristics. Herein, it may be possible to avoid unwanted reduction of the optical characteristics of the optical transmission element by providing a bank keeping the low melting point glass in the metal holder, wherein it is possible to actualize the condition in which the low melting point glass is eliminated from the periphery of the optical transmission element when the optical transmission element joins the metal holder. However, when a clearance portion between the optical transmission element and the metal holder is filled with the low melting point glass, the optical characteristics must be deteriorated.
When the optical transmission element joins the metal holder by using the joining material such as the low melting point glass, in other words, when the low melting point glass is affixed between the optical transmission element and the metal holder, cracks and double refraction may occur in the optical transmission element due to the stress based on the difference between the thermal expansion coefficients of the prescribed parts, which raises a problem in that a light extinction ratio must be deteriorated. This problem may be solved by using the low melting point glass whose thermal expansion coefficient approximates the thermal expansion coefficient of the optical transmission element. However, due to tensile stress caused by the combination of materials, cracks or separations may easily occur on the surface of the optical transmission element.